Respiratory modulation of cardiac time intervals.

To determine the effect of respiration on systolic and diastolic time intervals, simultaneous phonocardiograms, carotid pulse tracings, M mode echocardiograms, and respiratory curve tracings were measured in 25 healthy subjects. The positioning of each cardiac cycle in relation to the phase of respiration was assessed and the dependency of heart rate and cardiac time intervals on respiration was examined. Heart rate clearly varied over the respiratory cycle.